1. Field of the Invention
The present invention relates to a circuit for preprocessing a received ghost cancellation reference (GCR) signal in a received television signal, and then reducing television ghosts based on the enhanced GCR signal.
2. Description of the Related Art
In modem television transmission, echoes or ghosts have been one of the major problems. A multi-path phenomenon occurs when a transmitted signal is reflected from static objects such as mountains and buildings, or from dynamic objects such as clouds and airplanes, causing what is known as ghosts. This phenomenon is generally visible in terrestrial transmissions over air, but it could also occur over cable transmissions. The received signal consists of the usually post-cursory but sometimes pre-cursory, and attenuated copies of the transmitted signals overlapping with the strongest signal from a main path.
To assist in signal restoration, the Japanese Broadcasting Technology Association (BTA) has adopted a GCR signal based on an 8-field sequence of a time integral of a windowed [sin (x)/x] or sinc pulse. This same signal is also described in Section 1.1 of Annex 1 of Recommendation International Telecommunication Union Radio (ITU-R) BT.1124-3 as GCR Signal A. The GCR signal is transmitted on lines 18 and 281 of the 525-line television system. The sinc pulse extracted from the windowed sinc GCR signal has a flat frequency domain response desirable for channel compensation and equalization. However, U.S. Pat. No. 5,121,211 to Koo discloses that this BTA GCR signal suffers a performance limitation in high noise situation due to its low energy, and thus additional processing is required in this case.
U.S. Pat. No. 4,947,252 to Kobayashi, et al. applies Fourier transform to characterize the channel in the frequency domain for nearby and normal ghost cancellation. This processing circumvents the low energy characteristics in the time domain GCR signal in a high noise condition. However, the additional computation complexity and time is costly for tracking fast channel changes for this approach.
Further, U.S. Pat. No. 5,196,936 to Kobayashi, et al. suggested adding a plurality of sets of GCR signals together with noise level detection so that only when the ghost component exceeds the maximum detected noise level, should this component be considered effective for ghost cancellation. The problem with this solution is that is that at long intervals with sufficiently high estimated noise level, ghost detection will not feasible; thus, ghost cancellation will be rendered ineffective.
In order to solve the above problems, an advantageous solution is set forth in the present invention, in which the received GCR signal or the addition of a plurality of sets of GCR signals is first processed with a wavelet de-noising filter. Unlike a normal low pass filter where all details including edges are smoothed out, the wavelet de-noising filter smoothes out sufficient noise while preserving the edge information. This is important for obtaining the optimal waveform with differentiating the received GCR signal, from which ghost locations and levels are determined for the plurality of ghost cancellation filters that follows.